Natural organic matter (NOM) plays an important role in numerous natural and engineered systems due to its ability to associate with metal ions and organic compounds in solution and to sorb to mineral and other surfaces (e.g. Stevenson, F. J. (1982). Humus Chemistry—Genesis, Composition, Reactions. New York, John Wiley & Sons.; Leenheer, J. A. and J. P. Croue (2003). “Characterizing aquatic dissolved organic matter.” Environmental Science & Technology 37(1): 18A-26A.). Parameters and processes that are affected by the presence of NOM include the fate and transport of metals and organic contaminants in the environment, the environmental toxicity of organic and inorganic compounds, mineral weathering and metal leaching from soils, phytoremediation systems, scaling, and many more.
Humic substances represent an operationally defined, complex fraction of NOM with relatively high persistence in the environment due to their comparably low potential for microbial decomposition (MacCarthy, P. (2001). “The principles of humic substances.” Soil Science 166(11): 738-751; Stevenson, F. J. (1982)). They are described as a heterogeneous mixture of natural polyelectric acids in soils and waters that cannot be further classified based on their chemical structure (Gaffney, J. S., N. A. Marley, et al. (1996). Humic and fulvic acids and organic colloidal materials in the environment. Humic and Fulvic Acids: Isolation, Structure, and Environmental Role. J. Gaffney, S., N. A. Marley and S. B. Clark. Washington, D.C., American Chemical Society: 2-16).
Unfortunately, for many systems of interest, the environmentally relevant concentration levels of natural organic ligands are so low that their straightforward analytical quantification in experimental investigations is hindered. For instance, typical concentration ranges of humic substances in groundwater have been reported between 0.04 to 8.6 mg l−1 carbon (Thurman, E. M. (1985). Humic substances in groundwater. Humic Substances in Soil, Sediment, and Water. G. R. Aiken, D. M. McKnight, R. L. Wershaw and P. MacCarthy. New York, John Wiley & Sons: 87-103) and 0.1 to 10 mg l−1 carbon (Gaffney et al., 1996). Conventional analytical techniques for NOM quantification, such as dissolved organic carbon (DOC) analysis, UV-absorbance and fluorescence, lack sufficient sensitivity and/or exhibit dependence on solution conditions and organic ligand speciation (Northcott, G. L. and K. C. Jones (2000). “Experimental approaches and analytical techniques for determining organic compound bound residues in soil and sediment.” Environmental Pollution 108: 19-43; Novak, J. M., G. L. Mills, et al. (1992). “Estimating the percent aromatic carbon in soil and aquatic humic substances using ultraviolet absorbance spectrometry.” Journal of Environmental Quality 21: 144-147; Senesi, N. (1990). “Molecular and quantitative aspects of the chemistry of fulvic acid and its interactions with metal ions and organic chemicals. Part II. The fluorescence spectroscopy approach.” Analytica Chimica Acta 232: 77-106).